3.8 Sizing a Complete DWV System: Two Worked Walkthroughs

Sizing a Complete DWV System: Two Worked Walkthroughs

The order of operations

Every drainage sizing problem on this exam yields to the same five steps, in this order. Doing them out of order is the single most common way to arrive at a wrong answer that looks right.

1. Total the drainage fixture units from Table 702.1, using the correct private / public / assembly column. Use Table 702.1(1) for anything not listed, and Section 702.3 or Table 702.2 for pump and equipment flow.
2. Count the water closets and apply note 4 to Table 703.2 — 4 on any vertical pipe or stack, 3 on any horizontal branch or drain — before you look at capacity.
3. Fix the slope from Section 708.1: 1/4 inch per foot, with the 1/8 inch per foot exception available only for pipe 4 inches and larger with prior AHJ approval.
4. Read Table 703.2 in the correct row — vertical or horizontal — and check the maximum length column for vertical runs.
5. Verify the trap arms against the Iowa-amended Table 1002.2 and the one-diameter fall rule.

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Walkthrough 1: a two-story house

The fixtures

Second floor, full bath: one 1.6 gpf gravity-tank water closet, one lavatory, one combination bath/shower. First floor, half bath: one 1.6 gpf gravity-tank water closet, one lavatory. First floor, kitchen: one domestic kitchen sink with a food waste disposer. Basement: one domestic clothes washer standpipe, one 2-inch floor drain, and a condensate pump discharging 4 gpm continuously.

Step 1 — Total the fixture units (private column, Table 702.1)

Two traps in that table are worth pausing on. The clothes washer is 3.0 DFU, not 2.0 — a number people routinely under-count. And the condensate pump contributes 8 DFU, nearly a third of the house total, from a device most people would not think to count at all. Miss it and you have under-sized the building drain by a third of its load.

Step 2 — Water closets

Two water closets total. Both limits — 4 vertical, 3 horizontal — are satisfied on any single run in this house, so the count does not force a size.

Step 3 — Slope

The building drain is 3 inches or smaller in this problem, so the 1/8 inch per foot exception is unavailable. 1/4 inch per foot throughout.

Step 4 — Read Table 703.2

• Building drain and building sewer, carrying the full 25 DFU horizontally: a 3-inch horizontal drain carries 35 DFU. 25 is under 35, and only 2 water closets are involved, so 3 inches works. (Note that a 2-inch horizontal drain carries only 8 DFU — far too small.)
• The main stack, carrying the same 25 DFU vertically: a 3-inch stack carries 48 DFU and may run up to 212 feet vertically. A two-story house is nowhere near that. 3 inches.
• The second-floor bathroom branch: 3 + 1 + 2 = 6 DFU with one water closet. A 3-inch horizontal branch is required because it carries a water closet at all — the water closet's own trap arm is 3 inches, and drainage never reduces in the direction of flow.
• The kitchen sink branch: 2 DFU on a 1-1/2 inch trap. A 2-inch branch handles it (8 DFU horizontal capacity) and gives the disposer discharge room.
• The basement group — clothes washer 3 + floor drain 2 + condensate 8 = 13 DFU. A 2-inch horizontal branch carries only 8 DFU, so this group needs 3 inches.

Step 5 — Trap arms (Iowa Table 1002.2)

• Lavatory, 1-1/4 inch arm: minimum 2-1/2 inches to the vent, maximum 5 feet.
• Kitchen sink, 1-1/2 inch arm: minimum 3 inches, maximum 6 feet.
• Clothes washer and floor drain, 2 inch arms: minimum 4 inches, maximum 8 feet.
• Each water closet: developed length from the top of the closet flange to the inner edge of the vent may not exceed 6 feet.

The result: 3-inch building drain and sewer, 3-inch stack, 3-inch bathroom and basement branches, 2-inch kitchen branch.

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Walkthrough 2: a commercial restroom battery

The fixtures, public column: five 1.6 gpf flushometer-valve water closets, four wall-hung 1.0 gpf urinals with integral traps, four lavatories, and one service sink, all on one horizontal branch draining to a stack.

Step 1 — Total the fixture units (public column)

Step 2 — Water closets, and this is where the problem is decided

A 3-inch horizontal drain carries exactly 35 DFU, so on capacity alone the answer looks like 3 inches. It is not. Note 4 to Table 703.2 permits not more than 3 water closets on any horizontal branch or drain, and this branch has five. The branch must be 4 inches.

This is the single most reliable trap in UPC drainage sizing: the fixture-unit total lands right at a table limit, and the water closet count — a completely separate rule — overrides it.

Step 3 — Slope

The branch is now 4 inches, so the 1/8 inch per foot exception becomes available — but only where 1/4 inch per foot is impractical because of the depth of the street sewer or the structural arrangement of the building, and only with prior AHJ approval. Absent that, run it at 1/4 inch per foot.

Step 4 — The stack and the building drain

• Stack: 35 DFU vertically. A 3-inch stack carries 48 DFU, which covers it — but note 4 also caps a vertical stack at 4 water closets, and there are five. The stack is 4 inches. A 4-inch stack carries 256 DFU vertically and may run 300 feet.
• Building drain: carries the 35 DFU plus whatever else the building contributes, at 4 inches minimum, and never smaller than the branch feeding it.

Step 5 — Venting the battery

Under the Iowa-adopted UPC you vent this battery with individual and common vents, not with a circuit vent (Section 4.4). Each individual vent is not less than 1-1/4 inches and not less than one-half the diameter of the drain served, so a 3-inch water closet fixture drain takes at least a 1-1/2 inch vent. Size the collecting branch vent from the vent columns of Table 703.2 on total vent units and developed length, and keep not more than one-third of the permitted length horizontal.

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The five traps these two problems illustrate

1. The uncounted continuous flow. Two fixture units per gallon per minute adds up fast.
2. The clothes washer at 3 DFU, not 2.
3. The public column. A flushometer water closet is 3.0 DFU private but 4.0 public and 6.0 assembly.
4. The water closet count beating the fixture unit total. Check note 4 before you read the capacity row.
5. The slope exception that only exists at 4 inches and larger, and only with prior AHJ approval.